The present invention relates generally to a zoom lens and an electronic imaging apparatus using the same, and more particularly to an electronic imaging apparatus whose size is reduced, inclusive of a video camera or digital camera.
In recent years, electronic imaging apparatus (for instance, digital cameras) designed to take images of subjects using solid-state imaging devices such as CCDs or CMOSs in place of silver-halide film cameras have became mainstream. Further, they have now a wide spectrum of categories from the commercial high-end type to the compact low-end type. The invention takes aim at the compact low-end type in particular.
Users of such low-end type digital cameras would enjoy snapping shots over a wide range of scenes at any time in any place. For this reason, preference is now given to small-format digital cameras, especially ones of the type that can be well put away in the pockets of clothing or baggage, is convenient to carry around, and is slimmed down in their thickness direction. On the other hand, the compact type digital cameras have had generally a zoom ratio of the order of 3. Now, however, there is a growing demand for digital cameras having a zoom ratio higher than ever before to let them find use over a wider range of scenes.
The primary object of the invention is to achieve a zoom lens that is designed to make it easy to slim down an associated electronic imaging apparatus and have a zoom ratio of as high as about 5. As a matter of course, a particular object of the invention is to provide a less costly zoom lens that can take images with good image quality and lends itself to an electronic imaging device such as a CCD or CMOS.
Among prior compact zoom lenses having relatively high zoom ratios, there is a particular type comprising, in order from its object side, a positive first lens group, a negative second lens group, a positive third lens group and a positive fourth lens group, as already set forth in:
[Patent Publication 1]
JP(A)2002-31756, esp., Examples 1, 2, 3
[Patent Publication 2]
JP(A)2003-98433, esp., Examples 2, 3
[Patent Publication 3]
JP(A)2004-199000, esp., Examples 5, 6, 7
Of the sizes of a digital camera, the thickness direction size is primarily determined by a lens barrel; to slim down the digital camera, it is impeccable to slim down the lens barrel including a zoom lens. Consequently, of vital importance is the arrangement of a zoom lens optical system with the slimming-down of the lens barrel in mind. In more recent years, general use has been made of the so-called collapsible mount type where, when the digital camera is in use, the lens barrel is unfolded from within the digital cameral body, and when it is not in use, the lens barrel is folded down in the digital camera body. It is thus important that the zoom lens is set up while the slimming-down of the lens barrel upon folded down in the body is factored in.
Specifically, to slim down the lens barrel when it is folded down in the camera body, it is imperative to use zoom lens groups with a reduced number of lenses or curtail the total length of the zoom lens.
There is now also a mounting demand for the so-called low-cost digital cameras that allow for casual snapping and, at the same time, may be purchased offhand. Thus, there is an increasing demand for zoom lenses that satisfy slimming down and lower costs at the same time.
Such cost reductions may be achievable by the use of easily available vitreous materials; however, decreases in the number of vitreous material forming a zoom lens would be more easily achievable. A reduced lens parts count is vitally significant for cost, because not only can the vitreous material cost itself be cut down but also assembling can be much more facilitated.
However, there is no point in giving too much to cost reductions at the cost of the ability to flatter important for the camera's own performance as well as the “handy high-zoom-ratio zoom lens” that is most desirable for users. There is thus a mounting demand for a zoom lens that cuts down cost while making sure these specifications.
With the prior arts as mentioned above, the second lens group in particular that is especially of larger size in the thickness direction is designed such that its thickness direction size is reduced by allowing it to be made up of two lenses, a negative and a positive. At the second lens group, off-axis rays gain height from on the optical axis; when it is intended to make sure the necessary edge thickness of the lenses, their edges tend to grow very thick. Further, as the second lens group is made up of a lot more lenses, the entrance pupil position becomes far away from the object side in appearance, and so off-axis rays passing through the second lens group get much higher, resulting in the need of increasing the axial edge thickness to make sure the edge thickness. As a matter of course, a lot more lenses result in an increase in the axial thickness. Accordingly, as the lenses increase in number, it causes the diametrical size and axial thickness of this lens group to grow larger than necessary; even when they are stowed away in the lens mount, any sufficient compactness of the lens barrel is not achievable. Thus, composing the second lens group of as small as two lenses is preferable for the purpose of making the lens barrel compact.
In some prior arts, the total number of lenses in the zoom lens is reduced down to about 8 or 9, and that number could possibly provide a less costly zoom lens.
However, the prior arts as mentioned above have the following problems.
In the examples of Patent Publication 1, the number of lenses in the zoom lens is limited to up to 9: that number could possibly achieve cost reductions and a decrease in the total lens thickness.
However, that prior art has a zoom ratio of as low as about 3, a figure that would not well meet users' demands. All other prior arts, too, have a zoom ratio of about 3, a figure that would not be sufficient in view of specifications, although they may achieve cost reductions with fewer lenses.
Such problems as described above are caused for the following reasons.
Generally speaking, a zoom lens comprising a positive first lens group, a negative second lens group, a positive third lens group and a positive fourth lens group, the first and second lens groups have often some considerable role in the correction of optical performance in general, and chromatic aberration of magnification in particular. To minimize aberration fluctuations at these lens groups and make sure good enough optical performance, it is thus required that aberrations occurring in the first and second lens groups be as much reduced as possible.
To make good enough correction of chromatic aberrations, it is preferable to use a lot more lenses, but as the number of lenses increases, it does not only cause cost rises but also incurs an increase in the size of the whole zoom lens, because of the need of meeting fabrication requirements such as the optical axis thickness, the peripheral edge of each lens and the optical axis thickness of each lens.
In the prior arts as mentioned above, the first lens group is made up of one lens and the second lens group is made up of two lenses with this in mind. For this reason, however, there is no good enough optical performance ensured with the result that the zoom ratio still stays at about 3.